Actually for silver the longer the oxidation the blacker it gets.
Another consideration is the copper content, because when copper is involved, then the darker the oxidation gets as well. Copper often gets the green/blue/even multi-color patinas. However, when mixed with certain metals, the patina might change depending on the metal mix. Copper/tin mixes (bronzes) may yield a green patina, but can also give a dark brown to even black patina. This black patina can also be seen on silver/copper mixes where the mix has a large percentage of copper (in fact, silver/copper mixes with large amounts of copper often patina quicker than either silver or copper by themselves!
).
Hope this is now clear as mud...........
